1. Field of the Invention
The present invention relates to a conductive transparent probe and a probe control apparatus. More particularly, the present invention relates to a conductive transparent probe used in a tunneling luminescence microscope, and a probe control apparatus for controlling a distance between the apex of a probe and a sample, wherein the tunneling luminescence microscope measures optical and electronic characteristics of a very small region of a size of the nanometer order by detecting luminescence caused by applying a probe current into the sample.
2. Description of the Related Art
As devices become small and technologies for utilizing characteristics of individual molecules develop, great demands have arisen for technologies for characteristic evaluation of a very small region of a size of the nanometer order in materials (to be referred to as a nano region hereinafter), and for technologies for optical and electronic characteristic measurement of individual molecules intrinsically having a size of the nanometer order. For realizing such measurement and evaluation, a tunneling luminescence microscope (to be referred to as a TL microscope hereinafter) is provided that enables detection and analysis of luminescence caused by applying a current from an apex of a sharpened probe to a sample. In addition, a probe that is transparent and has conductivity (to be referred to as a conductive transparent probe hereinafter) has been developed, wherein the conductive transparent probe applies a current from its apex into a sample, and at the same time, receives and collects luminescence from the apex, so that luminescence collection yield is improved. The conductive transparent probe is powerfully used for characteristic evaluation of a nano region. As effectiveness of the TL apparatus for characteristic evaluation of a nano region increases, it is demanded by users that the sample to be measured is not only a material having only a conductive region but also a material in which a nonconductive region or a highly resistive region is mixed with the conductive region.
In an apparatus (to be referred to as a probe microscope hereinafter) that measures a sample by bringing a probe extremely close to the surface of the sample, it is very important to properly control a very small distance (to be referred to as a gap hereinafter) between the apex of the probe and the surface of the sample. Therefore, generally, as for the probe microscope (for example, a scanning tunneling microscope (to be referred to as an STM, hereinafter)) that utilizes a tunneling current flowing between the probe and the sample for measurement, a method of detecting the tunneling current flowing between the probe and the sample is used for controlling the gap (this control method is called an STM control method hereinafter). The reason for using this method for realizing precision gap control is that the tunneling current is very sensitive to the gap.
However, the STM control method can be applied only to a sample of which the whole region is electronically conductive, and the STM control method cannot be applied to a sample in which a nonconductive region or a highly resistive region is mixed. Therefore, a TL apparatus that enables gap control without using the tunneling current is desperately desired, such that the TL apparatus can be applied to a sample in which a nonconductive region or a highly resistive region is mixed.
As a gap control method without using the tunneling current, there is a method for utilizing an atomic force such as attractive force and repulsive force between the apex of the probe and the sample. In this method, when the apex of the probe approaches very close to the surface of the sample, atomic force between the apex and the surface is detected, and the gap is adjusted such that the detected value becomes constant.
For feeding back the detected value for performing gap control, there is a method of using an optical lever and a soft probe of a cantilever shape.
In this case, a laser beam is used for detecting a very small displacement of the probe. However, since the laser beam is extremely stronger than a detected signal light used for observing the sample, there is a problem in that the SN ratio decreases when measuring weak luminescence caused by the tunneling current.
It is desirable to use a leaner probe made of an optical fiber in order to suppress optical transmission loss in the probe. However, it is difficult to use such a probe as the soft probe of a cantilever shape that is necessary for realizing an optical lever.
In addition, there is a method called a shear force gap control method. In the method, a linear probe perpendicular to the surface of the sample is vibrated in a direction perpendicular to a center axis of the probe, so that atomic force is detected by measuring amplitude of the probe vibrating at a specific frequency. In this method, when a voltage is applied between the apex of the probe and the sample for causing luminescence, a current flows into a sensor used for detecting the amplitude, so that a detected signal is disturbed and gap control becomes unstable. Therefore, there is a problem in that a voltage cannot be applied between the probe and the sample when the shear force gap control method is used.